The present invention relates to systems and methods for controlling tracking systems and, more particularly, relates to systems and methods for controlling tracking systems with reduced residual jitter in the tracking system.
Telescopes used in many industries comprise large, sophisticated computer-controlled instruments with full digital outputs. And whereas telescopes have evolved over time, designers have paid particular attention to telescope parameters, including the light-collecting power of the telescope (as a function of the diameter of the telescope) and the angular resolution (as measured by image sharpness). For a perfect telescope operated in a vacuum, resolution is directly proportional to the inverse of the telescope diameter. In this regard, the perfect telescope generally converts a plane wavefront from distant star (effectively at infinity) into a perfectly spherical wavefront, thus forming the image with an angular resolution only limited by light diffraction.
In practice, however, errors such as atmospheric and telescope errors distort the spherical wavefront, creating phase errors in the image-forming ray paths. Generally, the cause of such atmospheric distortion is random spatial and temporal wavefront perturbations induced by turbulence in various layers of the atmosphere. Image quality can also be affected by permanent manufacturing errors and by long time scale-wavefront aberrations introduced by mechanical, thermal, and optical effects in the telescope, such as defocusing, decentering, or mirror deformations generated by their supporting devices.
In light of the errors introduced into such telescope systems, mechanical improvements have been made to minimize telescope errors. As a result of requirements for many large telescopes, typically those with primary mirrors above one meter, a technique known as active optics was developed for medium or large telescopes, with image quality optimized automatically by means of constant adjustments by in-built corrective optical elements. In this regard, telescope systems operating according to the adaptive optics technique generally include an adaptive optics assembly that comprises a deformable mirror that is optically coupled to the telescope behind the focus of the telescope at or near an image of the pupil. The deformable mirror, which includes a number of actuators for essentially changing the shape of the mirror, is controlled to apply wavefront correction to images received by the telescope.
In addition to the adaptive optics assembly, such telescope systems also generally include a tracking system. Whereas such conventional tracking systems are adequate in tracking objects imaged by the telescope system, such tracking systems have drawbacks. In this regard, the effectiveness of the closed-loop control of the tracking system in tracking the movement of the object is generally limited by the rate at which an imaging device can record an image received from the telescope system. For example, in a telescope system such as the 1.6 meter Gemini tracker, the tracking system includes an imaging device comprising a 128xc3x97128 speckle camera that has a maximum sample rate of 250 frames per second with an error rejection bandwidth of approximately 6 Hz. Because of the limit of the imaging device, some movement of the object, or residual jitter, of the object between each image taken by the focal plane array can escape the tracking system and cause degradation of images taken by the adaptive optics assembly.
In light of the foregoing background, embodiments of the present invention provide an improved optical tracking system, feed-forward augmentation assembly and method for controlling an optical imaging system, such as a telescope, capable of providing an image of a target. Advantageously, the optical tracking system, feed-forward augmentation assembly and method embodiments of the present invention are capable of modifying the reflector position drive signal with sensor data representative of a displacement position of the target. As such, the feed-forward augmentation assembly is capable of compensating for movement of the target that occur between instances in which images are received. By factoring in movement of the target, or residual jitter, between each image received, the optical tracking system, feed-forward augmentation assembly and method embodiments of the present invention can reduce the residual jitter that would otherwise cause degradation of images received by the optical imaging system.
According to one aspect of the present invention, a system is provided for controlling an optical imaging system capable of providing an image of a target. The system includes a closed-loop optical tracking system and a feed-forward augmentation assembly. The closed-loop optical tracking system comprises a reflector, an imaging device and a tracker controller. The reflector, which can be adjusted in at least one direction based upon images received from the optical imaging system, is capable of reflecting the image provided by the optical imaging system. In turn, the imaging device can record the image reflected by the reflector. And the tracker controller can generate a reflector position drive signal from a position of the target determined based upon the image recorded by the imaging device.
To reduce residual jitter in the reflector, the feed-forward augmentation assembly is capable of measuring a displacement of a position of the target. The feed-forward augmentation assembly can generate a feed-forward augmentation signal based upon the displacement measurement and the reflector position drive signal. More particularly, the feed-forward augmentation assembly can include a position sensor capable of measuring a current position of the target and a previous position of the target, such as based upon images received from the optical imaging system. In such embodiments, the feed-forward augmentation assembly can further include a beamsplitter capable of splitting the images received from the optical imaging system such that the position sensor receives a portion of the images and the reflector receives another portion of the images.
The feed-forward augmentation assembly can also include a signal processor that can then determine the displacement measurement based upon the current position of the target and the previous position of the target. In this regard, the signal processor can also be capable of generating the feed-forward augmentation signal based upon the displacement measurement and the reflector position drive signal. The signal processor can be capable of generating the feed-forward augmentation signal by determining jitter information based upon the reflector position drive signal and thereafter gain adjusting the displacement measurement based upon the jitter information. More particularly, the signal processor can be capable of comparing the jitter information based upon the reflector position drive signal with the jitter information based upon a previous reflector position drive signal. The signal processor can then be capable of gain adjusting the displacement measurement based upon the comparison.
With the feed-forward augmentation signal, then, the feed-forward augmentation assembly can combine the feed-forward augmentation signal and the reflector position drive signal to thereby drive the reflector to a position. In this regard, the feed-forward augmentation assembly can also include a summer capable of combining the feed-forward augmentation signal and the reflector position drive signal. Thus, the feed-forward augmentation signal can adjust repeatedly between images and, thus, between reflector position drive signals, to reduce residual jitter.
A feed-forward augmentation assembly and method of controlling the optical imaging system are also provided.